Ajax request was not successful.

Hello, im running isp config on a mail server, it was ok... i never had problems..
i have about 300 emails acconts added by ispconfig..

it worked very welll since i installed it, about 7 months ago.

Last week i connected to the web inteface to add a new email account...
some functions are not working and giving this error: Ajax request was not successful.

but the mail accounts was working well so i ignored the problem, added the accounts and closed the web interface.

Today i connected on web interface as admin but all sections are not working and giving this error..
i did some tests like adding a new admin & logging with it... but the login works but all places give this error and not show nothing to edit, like mail accounts, dns entries and domains.

I dont know if it is a bug or not, but i saw a lot of people asking about this problem on forums, can someone pleas check it?